As technology has advanced to reduce the size of hardware, make it less expensive and more versatile, the number of communication terminals, particularly satellite terminals, has increased much faster than the available band width, particularly for satellite communications. As a result, it is imperative for available communication channels to be used as efficiently as possible, i.e., to have minimum band width. Minimum band width can be achieved by using spectrally efficient waveforms in the communication channel.
Improved spectral efficiency is always attained by increasing the modulation order, i.e., number of bits per symbol. However, the majority of communication channels presently in use employ hard-limiting transponders, to eliminate effectively schemes using amplitude modulation. While phase and frequency modulation techniques remain as viable alternatives, implementation considerations make orders higher than quarternary impractical, particularly for portable earth-bound equipment. While some of the quarternary approaches have excellent performance, implementation thereof is considerably more complex and expensive than binary schemes. In addition, the quarternary approaches are usually not compatible with the large number of binary phase shift key receivers presently available.
To reduce the band width of a channel, it is important for interference from adjacent channels to be minimized. Interference between adjacent channels can be injected in transmitting and receiving a signal, e.g., on both an uplink and a downlink in a satellite communication signal. It is therefore important to provide a signal having a power spectral density that is well contained in the band width of interest at both the transmitter, receiver and at a relay station, such as a satellite. For binary phase shift key like waveforms, this requirement eliminates any post-modulation filtering as a viable approach to achieving satisfactory spectral containment. Such post-modulation filtering imparts an amplitude modulation component on the original binary phase shift key waveform, such that the magnitude of the amplitude modulation component generally increases as the filter band width decreases. Hard limiters generally employed in repeaters, however, remove the AM component, and in the process restore spectral sidebands to virtually the same level as subsisted before filtering. The net effect of employing post-modulation filtering on a conventional, prior art binary phase shift key signal causes the signal derived from the repeater or by the receiver to be very nearly the same as binary phase shift key. The result of this post-modulation filtering is that the transmitted signal should have no amplitude modulation component, to prevent the limiter from producing unwanted spectral sidebands.
It is also important for detection efficiency to be at the same level as that of existing systems. If adjacent channel interference is reduced by use of improved spectral containment at the expense of detection efficiency the reduced channel interference would be for naught. It is also highly desirable for existing binary phase shift key receivers to be compatible with any new generation of transmitting methods which reduce adjacent channel interference and enable detection efficiency to be maintained.
It is, accordingly, an object of the present invention to provide a new and improved method of modulating a carrier and transmitting a suppressed carrier constant amplitude wave.
A further object of the invention is to provide a new and improved low band width method of modulating a carrier with binary coded data wherein the data are primarily contained in a narrow band width, to minimize interference between adjacent channels.
Another object of the invention is to provide a new and improved binary transmitting and modulating method that provides high detection efficiency, commensurate with existing systems, is compatible with existing binary phase shift key receiver equipment, and has high spectral containment in transmission, relaying and reception.